High-strength steel sheet and method for producing the same

Abstract

There is provided a high-strength steel sheet and a method for producing the same. The high-strength steel sheet has a specified chemical composition and a steel microstructure including, by area fraction, 75.0% or more tempered martensite, 1.0% or more and 20.0% or less fresh martensite, and 5.0% or more and 20.0% or less retained austenite. A hardness ratio of the fresh martensite to the tempered martensite is 1.5 or more and 3.0 or less, the ratio of the maximum KAM value in the tempered martensite in the vicinity of the heterophase interface between the tempered martensite and the fresh martensite to the average KAM value in the tempered martensite is 1.5 or more and 30.0 or less, and the average of ratios of grain sizes of prior austenite grains in the rolling direction to those in the thickness direction is 2.0 or less.

The invention claimed is: 
     
       1. A steel sheet having a chemical composition comprising, by mass %:
 C: 0.08% or more and 0.35% or less, 
 Si: 0.50% or more and 2.50% or less, 
 Mn: 2.00% or more and 3.50% or less, 
 P: 0.001% or more and 0.100% or less, 
 S: 0.0200% or less, 
 Al: 0.010% or more and 1.000% or less, 
 N: 0.0005% or more and 0.0100% or less, and 
 the balance being Fe and incidental impurities, 
 wherein the steel sheet has a microstructure comprising, by area fraction, 75.0% or more tempered martensite, in a range of 1.0% or more and 20.0% or less fresh martensite, and in a range of 5.0% or more and 20.0% or less retained austenite, 
 a hardness ratio of the fresh martensite to the tempered martensite is in a range of 1.5 or more and 3.0 or less, 
 a ratio of a maximum kernel average misorientation (KAM) value in the tempered martensite in a vicinity of a heterophase interface between the tempered martensite and the fresh martensite to an average KAM value in the tempered martensite is in a range of 1.5 or more and 30.0 or less, and 
 an average of ratios of grain sizes of prior austenite grains in a rolling direction to those in a thickness direction is 2.0 or less. 
 
     
     
       2. The steel sheet according to  claim 1 , wherein the steel microstructure further comprises, by area fraction, 10.0% or less bainite, and
 the retained austenite has an average grain size in a range of 0.2 μm or more and 5.0 μm or less. 
 
     
     
       3. The steel sheet according to  claim 1 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of:
 Ti: 0.001% or more and 0.100% or less, 
 Nb: 0.001% or more and 0.100% or less, 
 V: 0.001% or more and 0.100% or less, 
 B: 0.0001% or more and 0.0100% or less, 
 Mo: 0.01% or more and 0.50% or less, 
 Cr: 0.01% or more and 1.00% or less, 
 Cu: 0.01% or more and 1.00% or less, 
 Ni: 0.01% or more and 0.50% or less, 
 As: 0.001% or more and 0.500% or less, 
 Sb: 0.001% or more and 0.200% or less, 
 Sn: 0.001% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.100% or less, 
 Ca: 0.0001% or more and 0.0200% or less, 
 Mg: 0.0001% or more and 0.0200% or less, 
 Zn: 0.001% or more and 0.020% or less, 
 Co: 0.001% or more and 0.020% or less, 
 Zr: 0.001% or more and 0.020% or less, and 
 REM: 0.0001% or more and 0.0200% or less. 
 
     
     
       4. The steel sheet according to  claim 1 , further comprising a coated layer disposed on a surface of the steel sheet. 
     
     
       5. A method for producing the steel sheet according to  claim 3 , the method comprising, in sequence:
 heating steel; 
 performing hot rolling at a finish rolling entry temperature in a range of 1,020° C. or higher and 1,180° C. or lower and a finish rolling delivery temperature in a range of 800° C. or higher and 1,000° C. or lower; 
 performing coiling at a coiling temperature of 600° C. or lower; 
 performing cold rolling; and 
 performing annealing by letting a temperature defined by formula (1) be temperature T 1  (° C.) and letting a temperature defined by formula (2) be temperature T 2  (° C.):
   temperature  T 1(° C.)=960−203×[% C] 1/2 +45×[% Si]−30×[% Mn]+150×[% Al]−20×[% Cu]+11×[% Cr]+400×[% Ti]  (1)
 
 where [% X] indicates the component element X content (% by mass) of steel and is 0 if X is not contained, and
   temperature  T 2(° C.)=560−566×[% C]−150×[% C]×[% Mn]−7.5×[% Si]+15×[% Cr]−67.6×[% C]×[% Cr]  (2)
 
 
 where [% X] indicates the component element X content (% by mass) of steel and is 0 if X is not contained, 
 
 wherein the annealing includes, in sequence:
 retaining heat at a heating temperature equal to or higher than temperature T 1  for 10s or more, 
 performing cooling to a cooling stop temperature in a range of 220° C. or higher and ((220° C.+temperature T 2 )/2) or lower, 
 performing reheating from the cooling stop temperature to a reheating temperature of A or higher and 560° C. or lower, where A is a freely-selected temperature (° C.) that satisfies (temperature T 2 +20° C.)≤A≤530° C.) at an average heating rate of 10° C./s or more, and 
 performing holding at the temperature A for 10s or more. 
 
 
     
     
       6. The method for producing the steel sheet according to  claim 5 , wherein a rolling reduction in a pass before a final pass of a finish rolling in the hot rolling is in a range of 15% or more and 25% or less. 
     
     
       7. The method for producing the steel sheet according to  claim 5 , wherein a heat treatment is performed after the coiling and before the cold rolling, and the heat treatment includes performing cooling from the coiling temperature to 200° C. or lower, performing reheating, and performing holding in a temperature range of 450° C. to 650° C. for 900s or more. 
     
     
       8. The method for producing the steel sheet according to  claim 7 , wherein a coating treatment is performed after the annealing. 
     
     
       9. The steel sheet according to  claim 2 , wherein the chemical composition further comprises, by mass %, at least one selected from the group consisting of:
 Ti: 0.001% or more and 0.100% or less, 
 Nb: 0.001% or more and 0.100% or less, 
 V: 0.001% or more and 0.100% or less, 
 B: 0.0001% or more and 0.0100% or less, 
 Mo: 0.01% or more and 0.50% or less, 
 Cr: 0.01% or more and 1.00% or less, 
 Cu: 0.01% or more and 1.00% or less, 
 Ni: 0.01% or more and 0.50% or less, 
 As: 0.001% or more and 0.500% or less, 
 Sb: 0.001% or more and 0.200% or less, 
 Sn: 0.001% or more and 0.200% or less, 
 Ta: 0.001% or more and 0.100% or less, 
 Ca: 0.0001% or more and 0.0200% or less, 
 Mg: 0.0001% or more and 0.0200% or less, 
 Zn: 0.001% or more and 0.020% or less, 
 Co: 0.001% or more and 0.020% or less, 
 Zr: 0.001% or more and 0.020% or less, and 
 REM: 0.0001% or more and 0.0200% or less. 
 
     
     
       10. The steel sheet according to  claim 2 , further comprising a coated layer disposed on a surface of the steel sheet. 
     
     
       11. The steel sheet according to  claim 3 , further comprising a coated layer disposed on a surface of the steel sheet. 
     
     
       12. The steel sheet according to  claim 9 , further comprising a coated layer disposed on a surface of the steel sheet. 
     
     
       13. A method for producing the steel sheet according to  claim 9 , the method comprising, in sequence:
 heating steel; 
 performing hot rolling at a finish rolling entry temperature in a range of 1,020° C. or higher and 1,180° C. or lower and a finish rolling delivery temperature in a range of 800° C. or higher and 1,000° C. or lower; 
 performing coiling at a coiling temperature of 600° C. or lower; 
 performing cold rolling; and 
 performing annealing by letting a temperature defined by formula (1) be temperature T 1  (° C.) and letting a temperature defined by formula (2) be temperature T 2  (° C.):
   temperature  T 1(° C.)=960−203×[% C] 1/2 +45×[% Si]−30×[% Mn]+150×[% Al]−20×[% Cu]+11×[% Cr]+400×[% Ti]  (1)
 
 where [% X] indicates the component element X content (% by mass) of steel and is 0 if X is not contained, and
   temperature  T 2(° C.)=560−566×[% C]−150×[% C]×[% Mn]−7.5×[% Si]+15×[% Cr]−67.6×[% C]×[% Cr]  (2)
 
 
 where [% X] indicates the component element X content (% by mass) of steel and is 0 if X is not contained, 
 
 wherein the annealing includes, in sequence:
 retaining heat at a heating temperature equal to or higher than temperature T 1  for 10s or more, 
 performing cooling to a cooling stop temperature in a range of 220° C. or higher and ((220° C.+temperature T 2 )/2) or lower, performing reheating from the cooling stop temperature to a reheating temperature of A or higher and 560° C. or lower, where A is a freely-selected temperature (° C.) that satisfies (temperature T 2 +20° C.)≤A≤530° C.) at an average heating rate of 10° C./s or more, and 
 performing holding at the temperature A for 10s or more. 
 
 
     
     
       14. The method for producing the steel sheet according to  claim 13 , wherein a rolling reduction in a pass before a final pass of a finish rolling in the hot rolling is in a range of 15% or more and 25% or less. 
     
     
       15. The method for producing the steel sheet according to  claim 13 , wherein a heat treatment is performed after the coiling and before the cold rolling, and the heat treatment includes performing cooling from the coiling temperature to 200° C. or lower, performing reheating, and performing holding in a temperature range of 450° C. to 650° C. for 900s or more. 
     
     
       16. The method for producing the steel sheet according to  claim 6 , wherein a heat treatment is performed after the coiling and before the cold rolling, and the heat treatment includes performing cooling from the coiling temperature to 200° C. or lower, performing reheating, and performing holding in a temperature range of 450° C. to 650° C. for 900s or more. 
     
     
       17. The method for producing the steel sheet according to  claim 14 , wherein a heat treatment is performed after the coiling and before the cold rolling, and the heat treatment includes performing cooling from the coiling temperature to 200° C. or lower, performing reheating, and performing holding in a temperature range of 450° C. to 650° C. for 900s or more. 
     
     
       18. The method for producing the steel sheet according to  claim 15 , wherein a coating treatment is performed after the annealing. 
     
     
       19. The method for producing the steel sheet according to  claim 16 , wherein a coating treatment is performed after the annealing. 
     
     
       20. The method for producing the steel sheet according to  claim 17 , wherein a coating treatment is performed after the annealing.